1. Field of the Invention
The present invention relates to wiring design of a printed board, more specifically, to a method for checking a return path of a printed board and a CAD (computer aided design) apparatus for designing patterns of a printed board, which can take an EMC (electromagnetic compatibility) countermeasure for preventing the generation of unwanted emission noise from a printed board.
2. Background of the Related Art
In order to reduce unwanted emission noise from electronics, such as digital audio-visual equipment or information equipment, it is important to figure out a wiring path in a circuit board which may constitute a noise source. In a conventional CAD for a printed board, when detecting a wiring error, a person visually checks whether a wiring path of clock wiring pattern violates an EMC design condition in order to correct the violated point of the wiring path.
However, in the above-mentioned CAD for a printed board, since detection of an error in EMC design is dependent upon a person, there is a problem that leakage of detection is likely to occur and it takes much time to detect a error.
For this countermeasure, since recognition technology of pattern shapes is developed in CAD for a printed board, it is possible to detect a guard ground, that is a guard pattern with ground attribution, existing adjacent to a signal wiring. For a typical detection method, if there is a wiring section in which a guard ground does not exist within a defined guard ground discriminant value with respect to a signal wiring, an error is thought to occur.
JP-2002-16337-A is related art in this field of invention.
However, in the above-mentioned method for detecting a guard ground, as shown in FIG. 8, in a case of using a small defined guard ground discriminant value G, a splay portion 43 of a guard ground, around which a pad or a via hole is located for a signal wiring pattern 31, will be judged as an error of discontinuity of the guard ground.
In another case of using a large defined guard ground discriminant value G, on the other hand, a short erroneous discontinued portion 44 of the guard ground will be judged to be satisfactory.
To solve the above problem, it is an object of the present invention to provide a method for checking a return path of a printed board and a CAD apparatus for designing patterns of a printed board, which can check a return path without mistake more quickly than the conventional method.